Aqueous dispersions of polyurethanes or polyurethane ureas are known (cf., for example, Angewandte Chemie, 82, (1970) pages 53 to 63; U.S. Pat. Nos. 3,905,929 and 3,920,598). These dispersions are of a high quality.
Not least among the factors which contribute to this high quality is the fact that many of these dispersions are free from emulsifiers. They contain chemically incorporated hydrophilic centers which make the otherwise hydrophobic elastomers self-emulsifiable.
The hydrophilic centers incorporated in known water-dispersible polyurethanes and polyurethane ureas may be both salt-like, i.e., ionic groups, and also hydrophilic non-ionic groups.
The first of these two types of polyurethanes, known as "polyurethane ionomers", include both polyurethanes containing chemically fixed cations, i.e., in particular, chemically incorporated ammonium ions, and also polyurethanes containing chemically fixed anions (e.g., incorporated sulfonate or carboxylate groups). The second of these two types of polyurethanes, i.e., non-ionic water-dispersible polyurethanes, include, in particular, the polyurethanes and polyurethane ureas containing polyethylene oxide chains as described in U.S. Pat. Nos. 3,905,929 and 3,920,598.
The dispersions of these polyurethanes have different characteristic property spectrums, depending upon the type of hydrophilic center. Thus, polyurethane ionomers dispersions are resistant to heat up to their boiling point because the solubility of the salt groups present in them is substantially unaffected by temperature. On the other hand, ionic dispersions coagulate when heated to only a moderate temperature (about 60.degree. C.) because the polyethylene oxide side-chains gradually lose their solubility in water at elevated temperature. Unlike ionomers, however, these dispersions are unaffected by the addition of substantially unlimited quantities of electrolytes and are even stable after freezing and thawing.
The sensitivity to electrolytes of cationic polyurethanes is particularly high. Polyurethanes containing quaternary ammonium or tertiary sulfonium groups dispersed in water coagulate immediately, even when only very small quantities of an electrolyte containing monofunctional ions, such as sodium chloride, in aqueous solution are added to them. Due to this property, considerable problems are involved in the production and use of cationic polyurethane dispersions. Thus, water which is substantially free from ions generally has to be used for the production thereof. "Hard" water undesirably coarsens the dispersed particles or leads to partial coagulation.
In many cases, cationic dispersions are difficult to pigment because ions adsorbed on the surface of the pigment make the pigment difficult to incorporate and give rise to inhomogeneities. The extreme sensitivity of cationic polyurethane dispersions to various types of additives has made them very difficult or even impossible to use on a commercial scale. On the other hand, cationic dispersions have particularly desirable properties, for example, extremely good adhesion to various substrates and an excellent film-forming capacity.
The present invention provides new water-dispersible cationic polyurethanes which, in the form of an aqueous dispersion, combine the advantage of excellent resistance to frost and electrolytes with the advantage of very high temperature stability.